1. Field of the Invention
The present invention relates to a displacement-measuring apparatus which has a stylus movable relative to an object for detecting the shape of the object, and also to a static-pressure bearing device designed for use in the displacement-measuring apparatus.
2. Description of the Related Art
A displacement-measuring apparatus for detecting the shapes of objects has an air bearing, a probe shaft, and a stylus. The air bearing supports the probe shaft, not contacting the shaft, such that the shaft can move in its axial direction. The stylus is attached to the distal end of the probe shaft, and put in rolling contact with an object, applying a predetermined contact pressure thereto. As the object is rotated, the stylus set in rolling contact moves back and forth in its axial direction. The stroke of the stylus is transmitted to and detected by the probe shaft.
FIG. 15 shows a conventional displacement-measuring apparatus A, which comprises a cylindrical housing X, an air bearing B located within the housing X, and a probe shaft C supported by the bearing B. The air bearing B has a through hole extending in its axial direction, the through hole having a rectangular cross section. The probe shaft C has a rectangular cross section and is loosely fitted in the through hole of the bearing B, such that it can move along its axis but cannot rotate around its axis.
The apparatus A further comprises a stylus D, a corner cube E, a core F, and a bias coil G. The stylus D is connected to the distal end of the probe shaft C. The core F is a hollow cylinder mounted on the proximal end portion of the shaft C. The corner cube E is fastened to the core F and, hence, to the proximal end of the probe shaft C, for detecting the displacement of the probe shaft C. The bias coil G is contained in the housing X and surrounds the core F, not contacting the core F.
FIG. 16 shows another conventional displacement-measuring apparatus H, which comprises a cylindrical housing I, an air bearing J located within the housing I, and a probe shaft K supported by the air bearing J under static pressure. The apparatus H further comprises a stylus L and a pin N. The stylus L is attached to the distal end of the probe shaft K. The pin N is set in screw engagement in a screw hole made in the wall of the housing I and protrudes into a U-groove M which is cut in the outer periphery of the proximal end portion of the shaft K and which extends parallel to the axis of the shaft K.
The air bearing J and the probe shaft K are spaced apart. Compressed air is supplied into the gap between the air bearing J and the shaft K from an air inlet port R through an air passage Q. The compressed air flows out of the gap and is discharged outside through an air outlet port S. The opening of the passage Q is adjusted by turning an adjustment screw (not shown).
In the displacement-measuring apparatus A, the contact pressure the stylus D applies to the object is controlled by changing the current supplied to the bias coil G. The larger the current, the more heat the coil G generates. There is the possibility that the heat impairs the reliability of the data acquired by operating the apparatus A for a long period of time. To reduce this possibility, a heat-radiating mechanism can be added to the displacement-measuring apparatus A. The use of such a mechanism renders the apparatus A complex and large.
The displacement-measuring apparatus A, shown in FIG. 15, has no sensors for the contact pressure applied from the stylus D to the object. Therefore, in the apparatus A it is impossible to adjust the current supplied to the bias coil G, minutely in accordance with slight changes in the contact pressure. This is detrimental to accurate detection of the shape of the object.
Further, as has been described, the probe shaft C has a rectangular cross section and is loosely fitted in the through hole of the bearing B which has a can move along its axis, but cannot rotate around its axis. Obviously, more labor and time are required to fit a bearing bush into the gap between the through hole and the shaft C and adjust this gap, than to place a bush into the gap between a circular hole and a shaft having a circular cross section and adjust the gap.
In the displacement-measuring apparatus H, shown in FIG. 16, the screw is turned to adjust the opening of the air passage Q. The rate at which the air flows outside through the air outlet port S is thereby controlled, thus adjusting the contact pressure the stylus L applies to the object. However, no measures are taken against changes in the characteristic of the air bearing J or changes in the contact pressure.
The pin N set in screw engagement in a hole made in the wall of the housing I is loosely fitted in the U-groove M cut in the outer periphery of the shaft K and extending along the axis of the shaft K. Hence, the pin N prevents the probe shaft K from rotating around its axis. The pin N hinders smooth moving of the shaft K. Due to the friction between the pin N and the shaft K, the shaft K may fail to move faithfully to the motion of the stylus L.